The present invention relates to a digital still video camera in which a still image is stored as digital data, and particularly relates to a digital still camera which consumes low electric current.
Recently, a digital still video camera in which an image is stored in a semiconductor memory such as a memory card, has been used instead of a still video camera in which a still image is recorded in a video floppy disk.
At the outset, referring to FIG. 3, the outline of a conventional digital still video camera will be explained as follows. The numeral 1 is a SSG which impresses the clock of the frequency f.sub.clk upon each portion of the digital still video camera. The numeral 2 is a CCD which functions as a light receiving unit which receives light from a photographic object and conducts photoelectric transfer so that an image signal can be generated. The numeral 3 is an A/D converter which converts the image signal sent from CCD 2 into digital image data. The numeral 4 is a frame memory which temporarily accumulates the digital image data. The numeral 5 is a digital processing circuit which converts the digital image data into the signals of Y, I, Q. The numeral 6 is a data compression circuit which compresses the data of signals Y,I,Q so that the amount of data can be reduced. The numeral 7 is a memory which stores the digital image data sent from the data compression circuit. This memory 7 is composed of a semiconductor memory provided in the digital still video camera or composed of an IC card (a card memory) which can be attached to and detached from the camera. The numeral 10 is a CPU which controls each unit. The numeral 11 is an operation panel which is used for the operation and changeover of various operation modes such as one shot photographing and serial shot photographing. The numeral 12 is a release switch.
The motions of the digital still video camera composed in the way described above are as follows.
When the serial shot photographing mode is selected by the operation panel 11 and the release switch 12 is pressed, the output signal of the CCD sensor 2 is converted into the digital image data at the A/D converter 3 and stored once in the frame memory. After that, the output signal is converted into the signals Y,I,Q at the digital procession circuit 5. Then, this is compressed by the data compressing circuit 6 and recorded in the memory 7. While the release switch 12 is pressed, these sequential motions are continuously conducted.
On the other hand, when the one shot photographing mode is selected at the photographing mode in the operation panel 11, and the release switch 12 is pressed, the output signal of the CCD sensor 2 is converted into the digital image data in the A/D converter 3 and stored once in the frame memory. These motions are conducted for each frame.
In the case of the above-described digital still video camera, when the serial shot photographing of 10 frames/sec is conducted, it is necessary to complete the process from picture-taking to the final processing in 100 msec per one sheet. In order to read out the data from the CCD sensor and store it in the frame memory, 33.3 msec (1/60 sec) is needed out of 100 msec, so that the signal conversion (the conversion from the image signal to the signals Y,I,Q) and the data compression must be conducted in the residual 66.7 msec. When the number of pixels of CCD is 400,000 (H800.times.V500), the signals Y,I,Q becomes as follows at the digital processing unit.
Y=800.times.500=0.4M byte PA0 I=400.times.500=0.2M byte PA0 Q=400.times.500=0.2M byte
Namely, one image is converted into the data of 0.8 m byte. Accordingly, 66.7 msec/0.8M byte=83.4 nseC/byte. This shows that the data needs to be processed in 83.4 nsec per byte.
When the data is processed at a high speed, CMOSIC is used and clock drive is conducted so that the processing can be completed in the processing time described above. The clock frequency is the same in one shot photographing and serial shot photographing.
The consumptive electric power is increased in proportion to the clock frequency, so that electric power not less than 1 W is spent in the processing circuit and the data compression circuit.
When driving is conducted by a high frequency in the case of one shot photography in which high speed processing is not necessary, it is not effective in view point of consumptive electric power. Especially, when the consumptive electric current is high in the case of a battery, the battery is consumed quickly due to the internal resistance. In other words, even when the consumed electric power (voltage X current X time) is the same, the efficiency is lowered in the case in which a large amount of current is flown for a short period of time as compared with the case in which a small amount of current is flown for a long period of time.